The Great Kyz Kala — 'Maiden's Castle' — is one of the most photographed structures on the Merv site, a rectangular mud-brick keep with massive corrugated walls. The vertical fluting is unusual: a structural and decorative response to the local mud-brick technique that gives the walls a strange ribbed silhouette and shadow pattern in the low desert light. It is also the most distinctive surviving example of Khorasanian Sassanid-era architecture (5th–7th century AD), though parts of the building were reused into later periods.
The Little Kyz Kala stands a few hundred metres west and uses the same corrugated technique on a smaller scale. The fortresses sit alone on the plain west of Sultan Kala and are particularly atmospheric in the early morning when the corrugated shadows fall sharpest.